1. Field of the Invention
The invention is directed to the field of spray nozzles and, more particularly, to a dual fluid spray nozzle adapted to produce a finely atomized spray of liquids.
2. Description of the Related Art
In many liquid spraying applications, it is desirable to produce finely atomized droplets of a liquid reagent. For example, in semi-dry scrubbing systems used to remove harmful gases such as acid flue gases produced by the burning of coal or of wastes, small droplets of a controlled size distribution optimize the mixing of the reagent and the flue gases and maximize performance of the gas cleaning process. Small droplets also evaporate more readily and minimize the dimensions of the reactor chamber in which the liquid is sprayed, while the accumulation of corrosive substances on the reactor walls is avoided.
The known dual fluid spray nozzles are generally unable, however, to produce finely atomized droplets of liquids without experiencing a number of technical problems. In a nozzle, the diameter and the corresponding cross-sectional flow area of the fluid passages affect the size distribution of the atomized droplets. The finer are the flow passages, generally the finer are the sprayed droplets. Accordingly, the diameter of the passages has been reduced in the known dual fluid spray nozzles in an effort to decrease the average size of the atomized droplets and produce a finely atomized spray.
This approach to producing a finely atomized spray has been inadequate for several reasons. For the atomization of slurries, reducing the diameter of the fluid passages causes a corresponding increase in the rate of clogging of the passages by the slurry particles. The reduced diameter passages effectively filter the particles and limit the maximum size of particles which can physically pass through them. Clogging is a fundamental problem associated with the atomization of slurry materials, even though, for most liquids, suspended solids are always present and may occasionally cause clogging.
Accordingly, selecting the size of the flow passages in a spray nozzle involves a balancing of the acceptable droplet size distribution against the acceptable rate of clogging of the nozzle. For slurries, clogging is so severe that it is not possible to achieve the desired droplet size distribution using the known dual spray nozzles as the necessary flow passage diameter is too small to be functional.
In addition to their clogging characteristics, slurry materials are also erosive and corrosive to the conventional materials used to construct spray nozzles.
In order to reduce the clogging of nozzle passages during slurry spraying operations, it is theoretically possible to increase the velocity of the atomizing fluid and the entrained slurry particles. Although this solution theoretically reduces clogging, at least when the slurry particles are smaller than the diameter of the passages, it is inadequate because increasing the velocity simultaneously increases the erosion rate of the passages. Therefore, the practical upper limit of the operating velocity is based on the acceptable level of wear of the nozzle. If erosion is too severe at the velocity necessary to prevent clogging, then such velocity is economically infeasible due to the shortened service life of the nozzle and the corresponding increased replacement costs.
Furthermore, the atomization of slurries using dual fluid spray nozzles is energy intensive, and increasing the velocity of the atomizing fluid only further increases energy usage as it increases the amount of energy required to input the atomizing fluid and slurry into the nozzle.
Therefore, in view of the inadequacies of the known dual fluid spray nozzles, there has been a need for a dual fluid spray nozzle which is capable of producing a finely atomized spray of a slurry at a reduced energy demand, and of producing a finely atomized spray at a reduced rate of erosion of the nozzle.